1. Field of the Invention
This invention relates to a novel chemically amplified resist material suitable for microfabrication.
2. Prior Art
Deep-ultraviolet lithography, one of a number of efforts currently being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, is thought to hold particular promise as the next generation in microfabrication technology. Deep-UV lithography is capable of pattern generation to dimensions of 0.3 .mu.m or less and, when a resist material having low light absorption is used, can form patterns with sidewalls that are nearly vertical to the substrate. One technology that has attracted a good deal of attention recently utilizes a high-intensity KrF excimer laser as the deep-UV light source. Resist materials with low light absorption and high sensitivity are needed to successfully apply this technology to large-volume production.
In light of this, acid-catalyzed chemically amplified positive resist materials were recently developed as disclosed in JP-B 27660/1990, JP-A 27829/1988, U.S. Pat. Nos. 4,491,628 and 5,310,619. Because of their excellent properties including sensitivity, resolution and dry-etching resistance, they are especially promising as resist materials for deep-UV lithography.
Chemically amplified resist materials, however, suffer from a post-exposure delay (PED) problem and a footing phenomenon. The PED problem is that in lithographic process, line patterns would have a T-top profile, that is, patterns become thick at the top if the leave-to-stand or delay time from exposure to post-exposure baking (PEB) is extended. The footing is a phenomenon that a pattern on a basic substrate, especially silicon nitride or titanium nitride substrate becomes widened in proximity to the substrate. It is believed that the T-top profile arises because the solubility of resist film is reduced in proximity to its surface whereas the footing arises because the solubility of resist film is reduced in proximity to the substrate. There also occurs a problem that dark reaction of eliminating acid labile groups proceeds in a PED duration from exposure to PEB, reducing the dimension of lines to be left. These problems are serious enough to prevent chemically amplified positive resist materials from practical application. Moreover, these problems not only complicate dimensional control in the lithographic process, but also adversely affect dimensional control in the processing of substrates using dry etching. In this regard, reference is made to W. Hinsberg et al., J. Photopolym. Sci. Technol., 6 (4), 535-546 (1993) and T. Kumada et al., J. Photopolym., Sci. Technol., 6 (4), 571-574 (1993).
It is understood that in these resist materials, air-borne basic compounds largely participate in the PED or T-top profile problem and basic compounds on the substrate surface largely participate in the footing phenomenon. Light exposure generates acid at the resist surface which is deactivated through reaction with air-borne basic compounds. As the leave-to-stand or delay time from exposure to PEB is extended, the amount of thus deactivated acid increases to retard decomposition of acid labile groups. An insolubilized layer is then formed at the resist surface, resulting in a T-top profile.
It is well known to blend basic compounds in resist materials to suppress the influence of air-borne basic compounds and to restrain the acid generated thereby from diffusing into unexposed portions, thereby improving PED, rectangularity and resolution.
For example, JP-A 289322/1993 discloses the addition of imidazole amines, JP-A 266111/1994 discloses the addition of imidazole, alanine, adenine, and adenosine, JP-A 120929/1995 discloses the addition of amines having pKa 6 or lower, JP-A 134419/1995 discloses the addition of pyridine compounds, and JP-A 128859/1995 discloses the addition of polyvinyl pyridine.
Through extensive investigations, we found that bases with high pKa were more effective for complementing for acids and hence, for improving the PED, eliminating the footing on basic substrates, and improving the resolution. In contrast, aromatic amines generally having low pKa were found little effective.
However, it is not true that higher pKa leads to better results. Various alkylamines as described in JP-A 149640/1988, 232706/1993 and 92678/1995, proton sponge known as a ultra-strong base disclosed in U.S. Pat. No. 5,609,989, 1,5-diazabicyclo[4.3.0]-5-nonene (DBN), 1,8-diazabicyclo[5.4.0]-7-undecene (DBU), cyclic alkylamines or quaternary amines such as tetramethylammonium hydroxide, some of which are shown below, were not fully effective. ##STR2##
In the case of negative working resist materials, there arise different problems from the problems of positive working resist materials. For example, PED problems of negative resists are that the top of a pattern is rounded due to film thinning and that a pattern on a basic substrate is thinned near the substrate, resulting in an undercut profile. Such an undercut pattern will readily turn down. The rounded or undercut portion of the pattern is formed by the progress of dissolution due to retarded crosslinking reaction.